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March 20, 1962 E. 6. DE MOTT ETAL 3,026,478

ELECTRICAL INSTRUMENT WITH A PERMANENT MAGNET CORE Filed June 23. 1959 2 Sheets-Sheet l ELMER G. DGMOTK CHARLES E. .STEGNER and GERALD STOLAR 2 INVENTORS TTOR/VE Y March 20, 1962 E. G. DE MOTT ETAL 3,026,478 ELECTRICAL INSTRUMENT WITH A PERMANENT MAGNET CORE Filed June 23. 1959 2 Sheets-Sheet 2 22; I I. I.

I r Jr ELMER GZDOMOTT, CHARLES E. STEGWER and GERALD STOLAR INVENfORS United States Fatent 3,026,478 ELEQTRICAL INSTRUMENT WITH A PERMANENT MAGNET CQRE Elmer G. De Mott, Chatham, Charles B. Stegner, Verona,

and Gerald Stolar, Linden, N.J., assignors to Daystrom,

Incorporated, Murray Hill, N.J., a corporation of New Jersey Filed June 23, 1959, Ser. No. 822,349 Claims. (Cl. 324-151) This invention relates to an electrical instrument and more particularly to a moving coil electrical instrument of the permanent magnet core type.

Moving coil instruments having a. permanent magnet core are well known in the art and comprise a transversely magnetized, generally cylindrical-shaped core surrounded by an annular soft iron yoke which acts as a return path for the magnetic flux. An annular air gap is defined by the outer periphery of the core magnet and the inn-er periphery of the soft iron yoke. A movable coil is arranged to rotate, in the said annular air gap, the coil being pivotally carried in bearings supported in bearing supporting arms, or brackets adjacent the opposite ends of the core magnet. The bearing brackets in the instrument of our invention comprise an integral part of a movement bracket which extends through the annular yoke. The core magnet, soft iron yoke and movement bracket may be held together as a subassembly by a single locking screw which is threaded to the yoke and engages the magnet. Novel means are provided on the movement bracket and cooperating members for accurately and precisely relatively locating the magnet, yoke and bracket whereby, even if the locking screw becomes slightly loose, as occasionally happens in use, the means for locating the magnet, yoke and bracket function to limit relative shifting between such parts.

In core magnet instruments, the magnet is often provided with a pair of pole pieces, or shoes, of soft iron which provide some compensation for irregularities in the flux density at the polar surfaces of the magnet which may arise from variations in the hardness or composition of the magnet at different regions thereof. In the instrument of our invention, the core magnet is provided with thin pole pieces; the pole pieces being thin to provide for a permanent magnet having a maximum magnet length. The permanent magnet also has a fiat formed thereon at the magnetic axis to provide an air space between the magnet and one of the soft-iron pole pieces, which air space increases the reluctance of the magnetic path and results in a more uniform flux at the pole pieces. Further, the magnetic axis of the core is located about 14 above the center-scale position of the coil, as measured in the direction of up-scale movement of the coil. The combination of pole shoes, a fiat on the core magnet at the magnetic axis, and the positioning of the flat above the center-scale position of the core provide means whereby a substantially uniform scale characteristic may be obtained.

In the instrument of our invention the pole shoes, or pole pieces, of the magnet assembly and the magnet are of maximum effective height without the necessity of rounding the corners of the pole pieces to provide clearance for the inside corners of the coil frame upon which the movable coil is wound. The expense involved in rounding the pole piece corners is avoided by making the pole pieces of a lesser height than the magnet whereby top and bottom surfaces of the magnet and pole pieces are axially displaced. A full height magnet may, therefore, be employed which results in an increase in flux when compared to a core in which the magnet and pole pieces are reduced in height to provide clearance for the inner radii of the movable coil frame.

An object of this invention is the provision of an inname 2 strument magnetic system in which the component parts thereof are inexpensively manufactured and easily assembled and adjusted.

An object of this invention is the provision of an instrument magnetic system comprising a movement bracket having bearing supporting arms integrally formed thereon, the said movement bracket including means formed thereon for easily and accurately relatively positioning a movabie coil, a core magnet with pole shoes, and a soft iron yo e.

An object of this invention is the provision of a novel core magnet and pole piece construction for use in an electrical instrument with a uniform scale characteristic.

An object of this invention is the provision of an instrument magnetic system which includes a core magnet, movement bracket having bearing supporting arms integrally formed thereon, and a soft-iron yoke, which components are held together by a single locking screw and which will not separate even if the locking screw becomes slightly loosened.

These and other objects and advantages will become apparent from the following description when taken with the accompanying drawings. It will be understood, however, that the drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the appended claims.

In the drawings wherein like reference characters denote like parts in the several views:

FIGURE 1 is a plan view of an instrument mechanism embodying our invention removed from its case;

FIGURE 2 is an enlarged side elevational view of the mechanism shown in FIGURE 1 with the scale plate and yoke broken away for clarity;

FIGURE 3 is an enlarged sectional view taken on line 3-4 of FIGURE 2;

FIGURE 4 is an enlarged sectional view taken on line 4 4 of FIGURE 2 only showing the instrument pointer in full and a portion of the scale;

FIGURE 5 is an enlarged vertical cross-sectional view of the novel movement bracket of our invention;

FIGURE 6 is an enlarged fragmentary isometric view showing a boss and projections formed on the upper surface of the top bearing supporting arm of the novel bracket of our invention, for the proper positioning of the scale plate therewith;

FIGURE 7 is an enlarged side view of the permanent magnet core and movable coil; and

FIGURE 8 is an enlarged fragmentary side elevational view of a modified form of movement bracket and yoke embodying our invention.

Reference is first made to FIGURE 1 of the drawings wherein there is shown an instrument scale plate, designated by the reference numeral 10, which is provided with a suitably calibrated scale 11. A pointer 12 cooperates with the scale 11 and is adapted to be moved therealong. Referring now, also, to FIGURE 2, the instrument mechanism comprises a substantially cylindrical permanent magnet core 13 positioned within and spaced from a soft-iron yoke 14 of annular cross-section which provides a return path for the magnetic flux of the magnetized core and also serves as a magnetic shield for a. coil 16 Wound upon a coil frame 17 of the suitable nonmagnetic material. The wire-wound movable coil 16, which carries the pointer 12, has secured thereto pivot stafrs 15, 15 which are supported in conventional pivots or jewel bearings 18 and 18 which, in turn, are threadedly engaged in top and bottom bearing supporting arms 19 and 21, respectively. The bearing supporting arms comprise an integral part of a novel movement bracket, designated 22. The movable coil rotates through an angle which varies with the magnitude of the current flowing in the coil, the current being conducted thereto through the spiral hair springs 23, as is well known in this art. The spiral springs 23 also function to restore the movable coil to a normal zero position with no current flowing through the coil, as is also well understood by those skilled in this art.

Reference is now made to FIGURE 3 of the drawings wherein it will be seen that the core magnet 13 is provided with pole shoes or pole pieces 26, 26 made of suitable magnetic material, such as soft-iron. The pole shoes are centered on an axis extending through the center scale position of the coil in a symmetrical arrangement. The ferrous pole pieces are preferably made thin whereby a permanent magnet having a maximum magnetic length may be utilized. The soft-iron pole pieces provide some compensation for irregularities in the flux density at the polar surfaces of the permanent magnet which may arise, for example, from variations in the hardness or composition of the permanent magnet at different regions. The permanent magnet 13 is provided with a fiat 27 which is centered on the magnetic axis of the core magnet to thereby provide an air space between the magnet and the soft-iron pole pieces 26. The air space so provided increases the reluctance of the magnetic path and results in a more uniform flux distribution at the pole shoes.

The constructional features of the permanent magnet core, which include a flat formed thereon and pole shoes secured thereto, are combined With a third feature to provide for an instrument having a substantially linear scale. The third feature includes the rotation of the core, within the pole pieces, to locate the magnetic axis thereof, a suitable angle above the center scale position of the coil measured in the direction of the up-scale movement of the coil. Reference is made to FIGURE 4 of the drawings wherein the center scale position of the coil '16 is designated by a broken line CSCS while the magnetic axis of the core is designated by a broken line MM. In FIGURE 4, a clock-wise, or up-scale rotation of the magnetic axis MM of the core 13 from center scale position, line CSCS, of the coil 16, of about 14 is shown, it being understood, however, that the invention is not limited to this particular angular displacement. The novelty of the core construction involves the use of a magnet assembly in which a magnet, having pole shoes, is provided with a fiat thereon which is centered on the magnetic axis of the magnet and which magnetic axis is rotated above the center-scale position of the coil; that is, in the direction of the up-scale movement of the coil. The above constructional features result in an instrument having a substantially linear scale, or one on which the calibrations are uniformly spaced, without the use of expensive and intricate magnetic system contours or coil designs.

In the core construction, the pole shoes 26, 26 are preferably cemented to the magnet 13, although they may be soldered or otherwise suitably secured thereto. Likewise, the magnet 13 is preferably secured to the movement bracket 22 by cementing in order to facilitate the assembly thereof. It will here be understood, however, that the assembly of the instrument can be carried out with such cementing of the core to the bracket. Referring to FIGURE 3 and the longitudinal sectional view of the bracket 22 shown in FIGURE 5, it will be seen that the axially extending arm portion of the bracket is provided with a cradle-like member 29 having an arcuate surface 31 of the same radius of curvature about the instrument axis as the magnet 13 and against which surface the magnet abuts. The longitudinal sides 32, 32 of the magnet cradle 29 (as seen in FIGURE 3) abut the spaced longitudinal pole shoe edges to thereby precisely angularly position the magnet on the bracket 22.

The bracket 22 (FiGURE which is preferably diecast, but which may be made by any other suitable process, is provided with a pair of inwardly, generally radially extending projections 33, 33 adjacent the ends of if the cradle 29 between which the core magnet 13 snugly fits. Axial displacement of the magnet assembly is thereby prevented by the projections 33, 33 and accurate axial placement of the magnet assembly on the bracket is assured.

The bearing supporting arms 19 and 21, which are integrally formed on the movement bracket 22, extend radially in one direction only from the axially extending portion of the bracket (FIGURE 5). Tapped top and bottom bearing holes 36, 36 are formed in the bearing arms, which holes are easily accurately located with respect to the arcuate surface 31 of the magnet cradle 29 in the manufacture of the bracket. Since the core magnet 13 is accurately positioned on the bracket, in a manner described above, the accurately located bearing holes assure the accurate positioning of the pivot bearings 18, 18 (FIGURE 2) and hence, the coaxial location of the coil with the magnet axis.

In the assembly of the instrument, the core magnet 13 with the attached pole pieces 26, is preferably first cemented, or otherwise suitably secured to the bracket 22 containing the jewel bearings 18, 18. The coil 16 is then rotatably mounted within said bearings. Unlike most prior art constructions, the assembly which includes the bracket 22, magnet 13 and movable coil 16, and associated parts, such as the hair springs 23, may be inspected and the necessary adjustments made thereto without a surrounding yoke thereon to obscure and hamper such inspection and adjustment.

As seen in FIGURE 2, the radial length of the lower bearing arm 21 is less than the inside diameter of the yoke 14 whereby the yoke is easily slipped over the above-described assembly of the bracket 22, magnet 13, coil 16 and associated parts. In order to properly orient the yoke, an outwardly radially extending, positioning lug, or projection 37, is formed on the outside wall of the axially extending portion of the bracket 22, which projection cooperates with a radially extending hole 38 formed in the yoke. A locking screw 39 engages a tapped hole 41 diametrically opposed to the hole 38. The locking screw is tightened into clamping relation with the magnet between the pole piece 26 on the magnet to lock the yoke, magnet and bracket together. The lug 37 and cooperating hole 38 serve to properly locate the yoke on the bracket both axially and angularly. The outer surface 40 of the axially extending portion of the bracket 22 is curved to fit the inner diameter of the yoke for surface engagement between the yoke and bracket (see FIGURE 3). That is, the inner and outer side walls 31 and 40, respectively, of the bracket are arcuately curved with the center of curve: ture thereof at the instrument axis, and with the yoke locked in place with the bracket and magnet, the said core magnet and yoke are concentrically positioned.

The axial portion of the bracket 22 is enlarged at the forward end thereof as at 22', against which enlargement or projection the yoke may abut (FIGURES 2 and 5). For added bracket strength, a fillet-like portion 22" is formed between the enlargement and the axial length of the bracket, and the forward inner wall of the yoke 14 is chamfered, as at 42, to accommodate such fillet-like portion on the bracket.

In prior art instrument mechanisms employing a core locking screw threadedly engaging the instrument yoke, the core magnet is free to fall out of the yoke if the locking screw is slightly loosened thereby possibly greatly damaging the instrument moving coil system. In the instrument of our invention, the magnet 13 is preferably cemented to the bracket 22 and is thereby held captive even if the core locking screw 39 is loosened. Further, even if the bracket 22 and core magnet 13 are not cemented together in the assembly of the instrument, they are retained within the yoke 14 even if the locking screw becomes slightly loosened by reason of the cooperating axially extending lug 37 and hole 38. Hence, there is no resultant damage to the instrument even when th lQcking screw 39 becomes slightly loose. The instrument magnetic system, together with the bracket 22, is obviously easily disassembled by the removal, or sufiicient loosening, of the single locking screw 39, for disassembly of the instrument.

Reference is now made to FIGURE 7 of the drawings wherein it will be noted that the ferrous pole pieces 26, 26 extend less than the full height of the permanent magnet 13. Heretofore, in prior art instruments, the pole pieces and magnet are made of the same height, and in order to provide clearance space between the coil frame and pole shoes, the pole shoe edges at the top and bottom are rounded. In our mechanism, the cost of rounding the corners is eliminated by making the pole pieces of less height than the magnet. In this manner, maximum effective height of the magnet and pole pieces is obtained without the necessity of rounding the pole piece corners. The full height magnet results in an increase .in magnetic flux as compared to a core in which the pole pieces and magnet are reduced to provide clearance for the inner radius of the movable coil frame. The pole pieces with squared ends are easily manufactured by a punching, rolling or extruding process at a very low cost.

The novel instrument mechanism of this invention provides means for accurately positioning the scale plate 10 relative to the instrument axis and in the proper angular position. Reference is now made to FIGURE 6 wherein it will be noted that a large boss 46 is formed on the upper face of the top bearing supporting arm 19 and is located in a coaxial relation surrounding the tapped bearing hole 36 formed in the arm. A pair of radially extending projections 47, 47 are formed on the boss 46 for proper circumferential location of the scale plate 10 (see also FIG- URE 1). It will be seen that the scale plate is provided with a cut-out portion having the outline of the diagonally opposite rounded sides of the boss 46 and projections 47, 4-7, which cut-out portion snugly fits the said boss and projections to properly orient the scale plate. Although not shown in the drawings, the instrument mechanism may be anchored within a suitable instrument case by utilization of the annular groove 48 formed in the yoke 14, for example, and the properly oriented scale plate may also be attached to the case by suitable fastening elements which extend through the clearance holes 49, 49 (FIGURE 1) formed in the said scale plate. 2

With a die-cast bracket 22, it will be apparent that the scale plate positioning boss and projections, .the core cradle 29, projections 33 for axially locating the core, and the projection 37 for axial and circumferential location of the yoke on the bracket are all obtained at substantially no extra cost once the die for forming the bracket is produced. It will be apparent that the instrument assembly of our invention is amenable to low cost production due both to the low cost of manufacturing the parts and to the ease with which the parts may be assembled and tested.

Having now described our invention in detail, in accordance with the requirements of the Patent Office, various changes and modifications will suggest themselves to those skilled in this art. For example, as shown in FIGURE 8 of the drawings, another embodiment of our invention involves the use of a yoke 14a and a bracket 22a in which the relative axial position of the yoke and bracket is fixed by the positioning of the yoke between the enlargement or projection 22 at the forward end thereof and a projection 51 on the bracket which engages the bottom end of the yoke. In the construction of FIGURE 8, it will be apparent that the necessity for the locating hole 38 in the bracket yoke and the cooperating projection 37 on the bracket (as seen in FIGURE 2) are eliminated. It is intended that these and other changes and modifications shall fall within the spirit and scope of the invention as recited in the following claims.

We claim:

1. In a moving coil electrical instrument of the type including a magnetic system comprising a generally cylindn'cal-shaped core magnet and a magnetic yoke surrounding the said core magnet and spaced therefrom by a radial air gap, a separable movement bracket having an axial arm portion extending through the yoke in abutting engagement with the yoke and magnet, and locking means extending between the yoke and magnet at a point diametrically opposed to the axial arm portion of the movement bracket and clampingly seeming together the core magnet, yoke and bracket, cooperating means formed on the yoke and movement bracket relatively axially and angularly positioning the yoke on the movement bracket, and cooperating means on the movement bracket and core magnet relatively axially and angularly positioning the core magnet on the movement bracket, the said cooperating means preventing disassociation of the core magnet, yoke and movement bracket when the said locking means is only slightly loosened.

2. The invention as recited in claim 1 wherein the said cooperating means formed on .the yoke and movement bracket include means forming a hole in the yoke diametrically opposed to the said locking means, and means forming a generally radial outwardly extending projection on the movement bracket which cooperates with the said hole in the yoke.

33. The invention as recited in claim 1 wherein the cooperating means on the movement bracket and core magnet include a pair of spaced pole shoes secured to the magnet, the core magnet abutting the axial arm portion of the movement bracket between the pole shoes with spaced edges of the pole shoes abutting the bracket to relatively angularly position the magnet on the bracket, and a pair of generally radial inwardly extending projections on the axial arm portion of the movement bracket which pro jections cooperate with the upper and lower ends of the core magnet to relatively axially position the said magnet on the bracket.

4. The invention as recited in claim 1 including generally parallel, radially extending, upper and lower bearing supporting arms integrally formed at the ends of the axial arm portion of the movement bracket, a pair of coaxial bearing members attached to the said bearing supporting arms, a movable coil surrounding said core magnet and pivotally supported in the bearing members for angular movement about an axis extending through the said core, a pointer attached to the said coil and rotatable therewith, a scale plate mounted on the upper bearing supporting arm, a boss formed on the face of the upper bearing sup porting arm at the instrument axis, projections formed on the said upper bearing supporting arm and extending radially of the said boss, and means forming a cutout portion in the scale plate which includes side walls which closely abut the side walls of the said projections on the upper bearing arm and a portion of the side walls of the said boss to thereby accurately coaxially and angularly position the scale plate on the upper bearing supporting arm.

5. In a magnetic field structure for an electrical instrument, a transversely magnetized cylindrical core, a coil surrounding said core and pivotally supported for angular movement about an axis extending through the said core, pole pieces of magnetic material secured to the said core, the length of the said pole pieces being less than the length of the core whereby the pole pieces terminate a spaced distance from the ends of the core to provide clearance space between the inside corners of the moving coil and pole pieces.

6. An electrical instrument of the type including a generally cylindrical-shaped transversely magnetized core, pole pieces secured to the said core and symmetrically positioned with respect to the magnetic axis of the core, a magnetic yoke surrounding the said core and spaced from the pole pieces on the core by a radial air gap decreased as compared with that beyond said pole pieces, a coil surrounding said core and pivotally supported for angular movement about an axis extending through said core, the said core having a plane polar surface normal to the magnetic axis of the core whereby an air gap is formed between a pole piece and the core, the magnetic axis of the said core being positioned upscale from the centerscale position of the said coil.

7. The invention as recited in claim 6, wherein the upscale positioning of said magnetic axis is about 14.

8. In a moving coil, electrical instrument of the type including a magnetic system comprising a transversely magnetized generally cylindrical-shaped core magnet, a magnetic yoke surrounding the said core magnet and spaced therefrom by a radial air gap, 9. pair of spaced pole shoes secured to the magnet and with inner surfaces of cylindrical curvature corresponding with the engaged cylindrical outer surface of the magnet, means forming a plane polar surface on the magnet normal to the magnetic axis of the magnet whereby an air gap is formed between the magnet at the plane polar surface and the pole shoe secured to the magnet, a separable movement bracket having an axial arm portion extending through the yoke in abutting engagement with the yoke and magnet, locking means extending between the yoke and magnet at a point diametrically opposed to the axial arm portion of the movement bracket and clampingly securing together the core magnet, yoke and bracket, the core magnet abutting the axial arm portion of .the movement bracket between the pole shoes with spaced edges of the pole shoes abutting the bracket to thereby relatively angularly position the magnet on the bracket, a coil surrounding the said core magnet and pivotally supported for angular movement about an axis extending through the core magnet, the magnetic axis of the said magnet being positioned upscale from the center-scale position of the coil, to thereby provide for a scale on which calibrations are substantially uniformly spaced.

9. An electrical instrument of the type including a generally cylindrical-shaped transversely magnetized core, pole pieces secured to the said core and symmetrically positioned with respect to the core, a magnetic yoke surrounding the said core and spaced from the pole pieces on the core by a radial air gap, a coil surrounding said core and pivotally supported for angular movement about an axis extending through said core, the said core having a plane polar surface normal to the magnetic axis of the core whereby an air gap is formed between a pole piece and the core, the magnetic axis of the core being positioned upscale from the center-scale position of the said coil.

10. The invention as recited in claim 9 wherein the upscale positioning of the said magnetic axis is about 14.

References Cited in the file of this patent UNITED STATES PATENTS 1,022,795 McClair Apr. 9, 1912 2,650,349 Lamb Aug. 25, 1953 2,719,267 Kunz Sept. 27, 1955 2,834,942 Eggers May 13, 1958 2,901,702 Endlich Aug. 25, 1959 FOREIGN PATENTS 192,901 Great Britain Feb. 15, 1923 

